The present invention relates to receivers and, in particular, to a receiver for analog and digital signals.
For over half a century, television signals have been broadcast in accordance with standardized analog signal formats, such as the NTSC format in the United States, the PAL format in Europe, and the SECAM format in France. Analog television signal formats have inherent limitations that preclude further improvement of picture quality, however, those inherent limitations can be overcome by broadcasting television signals in a digital format. Happily, advances in the design and processing of digital integrated circuits of ever increasing complexity and capability has resulted in sophisticated digital signal processing becoming available in a practical form and at reasonable cost.
Present digital television signal broadcast formats include the Digital Video Broadcast (DVB) format in use in Europe and the Advanced Television Standards Committee(ATSC) formats, such as ATSC format A/53 in use in the United States. Unfortunately, the various systems proposed and/or adopted for transmission of television signals in digital format are not compatible with the existing processing of television signals in analog format. This incompatibility arises not only because one format is analog and the other digital, but also because the signal processing at radio frequencies (RF) and/or at intermediate frequencies (IF) required to properly receive the formats are also different, such as in required filtering. Filtering at IF is typically implemented in surface acoustic wave (SAW) filters and SAW filters for NTSC signals should have a vestigial-sideband filter characteristic and those for ATSC signals should have a root-cosine filter or a xe2x80x9croofingxe2x80x9d (flat) filter characteristic.
The conventional approach to a receiver for receiving television signals in both analog and digital formats have required dual SAW filters and dual IF amplifiers, one set for each format, thereby undesirably increasing the complexity and cost of the television receiver and possibly introducing electrical interference. One such prior art arrangement is shown in FIG. 1. RF television signals received at antenna 10 (which may include a cable TV signal input as well as an air wave broadcast signal input) are down converted to the IF frequency, typically about 44-45 megahertz (MHZ) by RF tuner 12 whose gain is controlled to amplitude limit the RF signal in response to RF automatic gain control (AGC) detector 14. Frequency synthesis, band switching and filter tuning of tuner 12 is controlled in response to control signals received via a conventional I2C control data bus from a micro-controller. ATSC SAW filter 20 selects the bandwidth of an ATSC format signal at the IF frequency, typically about a 6 MHZ bandwidth for an ATSC format signal, which is applied to IF amplifier 26 when switch 24a is closed. Similarly, an NTSC SAW filter 22 selects the bandwidth of an NTSC signal at the IF frequency, typically about a 5.75 MHZ bandwidth, for an NTSC format signal which is applied to IF amplifier 26 when switch 24n is closed. In this example, the complication of IF switches 24a, 24n is introduced to avoid plural IF amplifiers. The IF signal from IF amplifier 26 is down converted to baseband for conventional analog signal processing of NTSC analog format signals and is digitally processed for ATSC digital format signals, to reproduce the transmitted program as a displayed picture and sound program. The IF signal from IF amplifier 26 is also applied to automatic gain control (AGC) detector 16 for gain-controlling IF amplifier 26, and in conjunction with RF AGC detector 14, controlling the gain of tuner 12.
In the United States, for example, the change over from all-NTSC analog format broadcasting to all-ATSC digital format broadcasting will be phased in gradually over many years so as not to instantly obsolete the installed base of existing NTSC television transmitters and receivers. Initially, only a few of the available programs may be in digital format and only a few of the broadcast channels will be changed from NTSC analog format to ATSC digital format, and later additional programs and channels will be converted. This leads to a further problem with the prior art arrangement of FIG. 1 in that the operation of ATSC and NTSC selection switches 24a and 24n is undefined in view of the unknown and unpredictable program and channel broadcast patterns in any particular geographical area.
Accordingly, there is a need for a receiver that can receive both analog format and digital format signals without the added complexity of dual IF filters. In addition, it is desirable that such receiver automatically select whether the received signal is in analog format or in digital format.
To this end, a receiver of the present invention comprises a tuner for converting input signals to IF signals, the input signals being in either one of an analog signal format and a digital signal format, an IF filter having a passband for passing the IF signals, and an IF amplifier for amplifying the IF signals. A first signal processor processes the IF signals that are in the analog signal format and a second signal processor processes the IF signals that are in the digital signal format.
In another aspect of the present invention, a method of receiving a signal which may be in either one of an analog signal format or a digital signal format comprises:
receiving the signal as if it is in the analog signal format, and detecting a signal characteristic thereof unique to the analog signal format;
receiving the signal as if it is in the digital signal format, and detecting a signal characteristic thereof unique to the digital signal format;
responsive to the detected signal characteristics unique to the analog signal format and to the digital signal format, respectively, selecting one of the analog signal format and the digital signal format as a received signal format; and
receiving the signal in the selected one of the analog signal format and the digital signal format.